In a manufacturing process of a semiconductor device or a flat panel display (FPD), there has often been used a liquid process in which a processing solution is supplied to a target substrate (hereinafter, referred to as “substrate” or “wafer”) such as a semiconductor wafer or a glass substrate. An example of this process is a cleaning process for removing particles or contamination adhering to a substrate.
As a substrate processing apparatus for performing a process such as the above-mentioned cleaning process on a substrate, there has been used an apparatus including a multiple number of single-wafer liquid processing units and a transfer mechanism. The liquid processing units perform a process by supplying a processing solution to a front surface or front and rear surfaces of a substrate such as a semiconductor wafer while the substrate is held and rotated on a spin chuck. The transfer mechanism loads and unloads the substrate to and from these liquid processing units. Further, in order to suppress an increase in footprint of the apparatus and obtain higher throughput, the liquid processing units may be stacked in multiple layers.
In the substrate processing apparatus, a processing solution nozzle (hereinafter, referred to as “supply nozzle”) for supplying a processing solution (chemical solution) may be switchably connected to a multiple number of chemical solution supply systems via a mixing valve (see, for example, Patent Document 1). In Patent Document 1, a flow rate control mechanism corresponding to each chemical solution supply system is installed between each chemical solution supply system and the mixing valve.
Meanwhile, a flow rate control mechanism may include a differential pressure flowmeter provided on the way of a flow path through which a fluid flows; a valve for controlling a flow of the fluid; and a controller for controlling an opening degree of the valve (see, for example, Patent Document 2). In Patent Document 2, the controller controls the opening degree of the valve based on a preset flow rate and a flow rate of the fluid measured by the differential pressure flowmeter. The differential pressure flowmeter may include a round tube; a first pressure gauge placed on an upstream side of the round tube; and a second pressure gauge placed on a downstream side of the round tube. The first pressure gauge measures a pressure of the fluid introduced into the round tube. The second pressure gauge measures a pressure of the fluid discharged from the round tube.    Patent Document 1: Japanese Patent Laid-open Publication No. 2008-34490    Patent Document 2: Japanese Patent Laid-open Publication No. 2006-153677
However, the above-described substrate processing method for processing the substrate by using the processing solution has problems as follows.
As described above, the flow rate control mechanism controls the opening degree of the valve such that a measurement value of the flow rate measured by the differential pressure flowmeter is close to the preset flow rate. Further, by way of example, the differential pressure flowmeter includes a component such as a round tube to make a flow path narrow, the first pressure gauge on an upstream side of the component and the second pressure gauge on a downstream side of the component. The differential pressure flowmeter obtains a measurement value of the flow rate based on a measurement value of the first pressure gauge and a measurement value of the second pressure gauge.
Even if the measurement value of the first pressure gauge or the measurement value of the second pressure gauge is deviated from a true value due to an electrical fault in the first pressure gauge or the second pressure gauge, the differential pressure flowmeter may calculate a measurement value of the flow rate based on the deviated value. At this time, the flow rate control mechanism may control this deviated flow rate to be close to the preset flow rate. Accordingly, by way of example, even though it is displayed on the flow rate control mechanism that the controlled flow rate is equal to the preset flow rate, an actual flow rate may be further deviated from the preset flow rate.
If the measurement value of the flow rate is deviated from the true value as described above, it is difficult to gauge the true value of the flow rate unless a flow rate of the processing solution supplied through the supply nozzle is directly measured by a measuring cup. Therefore, if the measurement value of the flow rate measured by the differential pressure flowmeter is deviated from the true value due to, for example, an electrical fault in the first pressure gauge or the second pressure gauge, there has been a problem in that it is not easy to detect such deviation.
Meanwhile, there is a differential pressure flowmeter using a differential pressure sensor in which a pressure difference on an upstream side and a downstream side is measured by a diaphragm provided to shut a communication unit for communication between the upstream side and the downstream side. In this differential pressure flowmeter using the differential pressure sensor, there may be a case where a measurement value of a flow rate is deviated from a true value due to a bending or deformation of the diaphragm. In this case, there has been a problem in that it is not easy to detect such deviation.